The present invention relates to a cable including optical fibers surrounded by a sheath having longitudinal reinforcement.
In the cable industry, it is well known that optical fibers must not be subjected to mechanical stress if their transmission performance or their lifespan are not to be degraded.
Optical fibers are therefore disposed in a sheath which protects them mechanically against traction, longitudinal compression, and crushing forces.
Furthermore, in order to limit stress on the optical fibers both while the cables are laid and during temperature variations which cause the materials constituting the sheath to expand or retract, it is known to use reinforcement members extending in a layer of the sheath of the cable.
It is found that the resistance to crushing depends mainly on the thickness of the material forming the sheath. A cable-crushing force applied to the sheath over the reinforcement members is not absorbed sufficiently by the sheath and is therefore transmitted to the reinforcement members which run the risk, under said force, of causing the sheath to tear. This results in a risk of damage to the optical fibers.
Document FR-A-2 745 642 proposes an optical fiber cable having a sheath that includes four diametrally opposite projecting ribs extending along the length of the cable. Such a cable has good crushing strength. However, it poses sealing problems both when connected to a connection box and when laid in a conduit by using a flow of compressed air. Furthermore, as a result of its structure, such a cable is difficult to wind evenly onto a reel and slides with considerable friction in a cylindrical conduit.
Document DE-A-26 35 917 discloses a cable which includes optical fibers surrounded by a sheath whose cross-section has an oval outline presenting a minor axis. The sheath comprises a layer in which elongate reinforcement members extend, the reinforcement members being disposed on either side of the minor axis. The cable thus has greater crushing strength along the direction of the minor axis and, when the cable rests on a support, its oval outline causes the cable naturally to take up an orientation such that the minor axis extends perpendicularly to the support, i.e. in the direction in which crushing forces usually act. However, that cable is markedly oval in shape which makes sealing difficult at connection boxes. The oval shape also enables the cable to be wound evenly onto a reel and to slide with little friction in a cylindrical conduit.
An object of the invention is to propose an optical fiber cable which can withstand a significant amount of crushing, without affecting its bending properties or its suitability for sealed mounting at its end.
In order to achieve this object, the invention provides a cable including optical fibers surrounded by a sheath whose cross-section has a substantially oval outline presenting a minor axis, the sheath comprising at least one layer in which at least two elongate reinforcement members extend, the reinforcement members being disposed symmetrically on either side of the minor axis, the ovalization of the outline lying in the range 1% to 10%.
The ovalization is thus sufficient for the cable naturally to take up a position presenting greatest crushing strength in the appropriate direction when it rests on a support. Furthermore, in this position, the relative flexibility of the cable in various directions is such as to minimize the risk of the cable undulating while it is being pushed into place in a conduit. In addition, the cable ends are generally mounted in cylindrical endpieces with clearance such that the above-mentioned ovalization of the outline of the cable does not affect the possibility of providing a sealing of the inlet of connection or splice boxes.